Novel Use of Drivers
i posted exactly how to do this in your other post almost a month ago.

Can I use Weight Paint to Control Animation Properties?

The following code will work on an object i fractured using  Cell fracture, the object named "Cube". then i used the Bullet Constraint tools 'Single constraint' to make the constraints.
this code wont work if you have used Empties to constrain the objects together.

and a blend file.

https://dl.dropboxusercontent.com/u/14364171/constraint_control.blend 

press alt A, you will see the box stays together.
run the script (alt P in textblock) and then animate again. you will see the box stays together, until the plane moves above each object.

object_name = "Cube"


import bpy



for obj in bpy.data.objects:
    if  object_name+"_cell" in obj.name:
        driver = obj.rigid_body_constraint.driver_add('enabled')
        driver.driver.expression="this_obj&lt;mask_obj"
        driver_variable = driver.driver.variables.new()
        driver_variable.name = "this_obj"
        driver_variable.type = 'TRANSFORMS'
        driver_variable.targets[0].transform_type = 'LOC_Z'
        driver_variable.targets[0].id = driver_variable.id_data    

        driver_variable = driver.driver.variables.new()
        driver_variable.name = "mask_obj"
        driver_variable.type = 'TRANSFORMS'
        driver_variable.targets[0].transform_type = 'LOC_Z'
        driver_variable.targets[0].id = bpy.data.objects['mask']